One more test for implicit commodity pricing (test-for-failure)

tests/prices.test

@@ -161,3 +161,12 @@ bin/hledger -f - balance
                  16$
                 -10£
 >>>=0
+# 12. When commodity price is specified implicitly, transaction should
+#     NOT be considered balanced out when BOTH amounts are negative
+bin/hledger -f - balance
+<<<
+2011/01/01 x
+  a  -10£
+  b  -16$
+>>>
+>>>=1